A solar cell or photovoltaic (PV) cell is an electrical device that converts the energy of light directly into electricity by photovoltaic effect. Energy generated from solar cells offers renewable, environmentally friendly and readily available alternatives to fossil fuels. Typically a solar cell utilizes a PV layer made of semiconductor materials in the form of a p-n junction for energy conversion. Metal electrodes are laid on the front and the back of the semiconductor materials to conduct the produced voltage and current to external circuitry for power storage or transportation.
An array of solar cells can be interconnected and assembled into a solar module or a solar panel to achieve aggregated current and voltage generated by the individual solar cells. One prevalent approach of interconnecting solar cells is to overlap two solar cells to realize electrical connection, e.g., an upper cell and a lower cell. In a typical solar cell configuration, the back electrode of an upper cell is electrically connected with the front electrode of a lower cell. In this manner, multiple solar cells are interconnected in series.
More specifically, metal contacts disposed on the front and the back sides of a PV layer of a solar cell form the front electrode and the back electrode, respectively. The back electrode is disposed between the PV layer and a non-conductive substrate layer. Thus, when two cells partially overlap each other, the non-conductive substrate is disposed between the back electrode of the upper cell and the front electrode of the lower cell. To provide electrical continuity between the two overlapping cells, vias are made on the substrate and filled with a conductive material, which is commonly in the form of resin, paste or ink during the filling process and hardens after a cure process. Hereinafter, the vias on the substrate may be referred to as “back vias.”
In practice, a back via is usually subject to overfill with the conductive material to prevent the formation of a void inside the via which can potentially lead to a failed contact. However, filling a back via with excess conductive material tends to cause an uncontrolled lateral overflow (or smear) of the conductive material from the via, especially when the two solar cells are stacked and pressed together for integration. Unfortunately, many contributing factors make it difficult to determine a precise amount of the conductive material for a void-free via without resulting in overflow, such as the variations in the volume control capability of material placement, via size variations, and material property changes over time, temperature, moisture, contact surface, and etc. For example, for a reasonably repeatable automatic dispense or printing process, the dispensed volume normally varies by 5%. With material property changes over time, the volume variation can increase to 10%. The via sizes can also vary with the laser drilling process and substrate material properties.
The conductive material overflow in a solar cell can undesirably reach and bridge the front and back electrodes of another solar cell (e.g., the lower solar cell) and cause short circuit. Conventionally, to solve this issue, an insulating material is deposited around the perimeter of the solar cells followed by a cure procedure. Unfortunately, this contributes to significantly increased manufacturing cost and time.